1. Field of the Invention
This invention relates in general to instruments for detecting fluid pressures, and in particular to a Fabry-Perot optical interferometer which includes a metal cup pressure transducer for accurately monitoring downhole wellbore pressures over a range of operating temperatures.
2. Description of the Prior Art
Prior art pressure transducers have been proposed for monitoring downhole wellbore pressures in oil and gas wells. These pressures are monitored for determining downhole well conditions. By monitoring downhole wellbore pressures, conditions that are occurring within the wellbore and hydrocarbon bearing formations may be monitored so that production operations can be altered to maximize production of petroleum reservoirs.
Prior art electrically operated pressure transducers have been utilized. However, electrical and electronic pressure transducers present problems that arise when electrical signals are transmitted within wellbores. In general, there are many problems encountered in providing an insulated electrical conductor for transmitting electrical signals within wellbores. Electrical conductors are difficult to seal against exposure to wellbore fluids, which typically short electrical signals once they penetrate insulating materials around electrical conductors. Electrical conductors are also subject to corrosion and deterioration in harsh wellbore environments. Additionally, electrical transmissions are subject to electrical noises in some production operations.
Prior art optical interferometers have been proposed for measuring downhole wellbore pressures and temperatures. However, optical interferometers are typically very sensitive to temperature variations, and the downhole temperature of a specific position within a wellbore will change over time dependent upon different factors such as, for example, production rates, the types of fluids produced over the life of the well, and other downhole wellbore conditions. Additionally, it is difficult to determine what the precise downhole wellbore temperature will be at the position where an optical interferometer pressure transducer will be placed in the wellbore.
Prior art optical interferometer pressure transducers monitor pressures by passing an optical signal, or light, across a distance and measuring very small changes in that distance that occur in response to changes in pressure. Variations in temperatures in prior art optical interferometers can greatly affect and change the distance across which the light is passed for monitoring downhole wellbore pressures. Thus, prior art optical interferometers would be subject to erroneous readings due to changes in downhole wellbore temperatures.
Some prior art optical interferometer pressure transducers proposed for monitoring downhole wellbore pressures have utilized reference legs for comparing a measurement leg of an optical pathway to a reference leg, or reference optical pathway, which is disposed downhole within the wellbore with the measurement leg. However, inaccuracies arise due to differences between the measurement leg and a reference leg, and these differences are accurately compensated only at a specific downhole wellbore temperature.
Some prior art optical interferometer pressure transducers have attempted to utilize special materials to reduce the temperature coefficient of thermal expansion of pressure transducer members. However, since optical signals are used to measure a very small distance, very small variations in optical pathway distances can cause large errors in measured pressures, including those pressure transducers which utilize optical reference legs and those which utilize a Fabry-Perot type of measurement system.